In the microscopic differential diagnosis of fibrosing lung processes organic deposits are often encountered which defy identification by histological methods. Assessment of the aetiological significance of these deposits depends on an accurate analysis. Energy-dispersive X-ray microanalysis with an electron microscope provides the means to identify organic deposits in the tissues. Such analyses, which do not require elaborate techniques, show up the type and structure of the deposits and the topographical relation to the lung lesion. They provide detailed information for assessing pulmonary lesions caused by fibrosis-inducing dusts.